The problem for the GFX sensor/in-camera RAW file processing, seems to be that it is sensitive or unforgiving with regards to highlights, then ETTR is flawed/hindered since your only option is to turn down the exposure to avoid blooming etc,,, Sort of how it was in the beginning with Nikon D1
The sensor in the D810 seems to be extremely forgiving with regards to blown highlights,,, recovery in PP is mind boggling - To me it seems like Nikon has stumbled upon a sensor and in-camera RAW firmware processing Fine line that is extremely capable even though its a 36MP FF sensor.
We can hope Fuji is able to tweak the firmware on the GFX, but so far it doesn't look promising,,, all test I have seen has had these blocked shadow/dark areas more or less,,,
Sony has similar Fine line performance for the A7R II,,, really an impressive camera!